This is from the latest Arenanet blogpost.
Make sure to set your alarm clocks early!Pre-Purchase with 3-Day Headstart: Everyone who pre-purchased Guild Wars 2 will enjoy three full days of Headstart Access. The three-day Headstart Access will officially begin at 0:00 AM Pacific time (GMT -7:00) on August 25th. However, please note that in order to ensure that we’re fully prepared for that fateful hour we may bring servers online up to 3 hours prior. If you’re a player committed to getting in first to grab that character name of your dreams you’ll want to be keeping an eye on things during that time period.